Mechanical coolers are devices used for cooling, heating, and thermal transfer in various applications. For example, mechanical coolers are used to cool certain sensor elements, to cool materials during semiconductor fabrication, and to cool superconducting materials such as in Magnetic Resonance Imaging (MRI) systems. Mechanical coolers typically utilize a thermodynamic cycle (often involving the compression and expansion of a fluid) to shift heat and create cold portions that are useful for cooling. Cryocoolers are a class of mechanical coolers that can achieve cold temperatures in the cryogenic range (e.g., <˜123 K). Different types of mechanical coolers may comprise various valves, thermal compressors, mechanical compressors, displacers, etc., to bring about expansion and compression of the working fluid.
A pulse tube cooler includes a stationary regenerator connected to a pulse tube. A reservoir or buffer volume may be connected to the opposite end of the pulse tube via a phase control device such as a sharp-edged orifice or an inertance tube. The reservoir, pulse tube, and regenerator may be filled with a working fluid (e.g., a gas such as helium). A compressor (e.g., a piston) compresses and warms a parcel of the working fluid. The compressed working fluid is forced through the regenerator, where part of the heat from the compression (Qo) is removed at ambient temperature and stored at the regenerator. The working fluid is then expanded through the pulse tube and the phase control device into the reservoir. This expansion provides further cooling (Qc) that takes place at a cold temperature (Tc). The cooling occurs at a cold end of the pulse tube nearest the regenerator. A hot end of the pulse tube farthest from the regenerator collects heat.
Pulse tube cryocoolers do not have moving parts at the cold end, such as displacer pistons or valves. To achieve the desired cooling, the combination of the phase control device and the reservoir cause a phase shift between mass waves and pressure waves generated by the compressor. By restricting or slowing the mass flow to the buffer volume, the phase control device may serve to shift the phase of the mass flow relative to the pressure wave generated by the compressor.
Multistage pulse tube coolers are used to achieve temperatures colder than can be achieved with a single cooler alone. Multistage coolers can be arranged in series, where the cold end of the first cooler is connected to the hot end of the second pulse tube, or in parallel, where the cold end of the first stage is connected to the cold end of the second stage. Some load shifting between stages can be brought about by varying the frequency, charge pressure and/or temperature of each stage.